In recent years, optical disc devices and communication devices have been rapidly increased in capacity (speed) and reduced in size, and technologies thereof have made remarkable progress. In a conventional optical disc device, automatic gain control (hereinafter referred to as “AGC”) for keeping the amplitude of an input analog signal constant is carried out as follows. That is, the input analog signal is subjected to full-wave rectification or half-wave rectification, and the rectified signal is input to a variable gain amplifier (hereinafter referred to as “VGA”) to be automatically gain-controlled. Further, a storage medium having a sector format structure, such as a MO or a DVD-RAM, stores data having ID sections including information of data position (address information) and DATA sections including user data. Especially in a DVD-RAM, there exist blocks having plural pieces of address information which are called CAPA (Complementary Allocated Prepit Addresses) in the ID sections. The plural pieces of CAPA have different signal characteristics such as signal amplitudes and offsets when being reproduced.
Hereinafter, the conventional AGC control system will be described with reference to FIG. 15.
FIG. 15 is a block diagram illustrating an example of the conventional AGC control system. In FIG. 15, reference numeral 610 denotes an analog signal, 611 denotes a VGA for varying a gain on the basis of external control, 612 denotes a full-wave rectifier (FWR) for turning back the output signal from the VGA 611 at its center, and rectifying the signal to obtain amplitude information, and 613 and 614 denote low-pass filters (LPF) for removing high-frequency components from the full-wave-rectified signal. Reference numeral 630 denotes an ID gate signal which is a signal for distinguishing between the ID section and the DATA section.
Initially, the input analog signal 610 is given a gain by the VGA 611, whereby the amplitude thereof is changed. The output from the VGA 611 is full-wave-rectified by the FWR 612, whereby amplitude information is taken out.
Since the amplitude information outputted from the FWR 612 includes high-frequency components which are unnecessary for AGC control, the high-frequency components are removed by the LPF 613 and the LPF 614, and the output from the LPF is supplied to the VGA 611 as a gain control signal.
The LPF 613 and the LPF 614 are provided for performing independent AGC control for the DATA section and the ID section of the input analog signal 610, respectively, and switching therebetween is carried out by the ID gate signal 630. Because the DATA section and the ID section have considerably different signal characteristics, the respective sections are independently AGC-controlled.
As described above, in the conventional AGC control system, the ID sections are united, and the first CAPA signal and the second CAPA signal are AGC-controlled with the same gain. Therefore, the first and second CAPA signals with their signal amplitudes being varied are input to the subsequent processing system. As a result, CAPA reading accuracy is degraded, and address information reading errors occur frequently. Thus, many system retries occur at reading, resulting in a reduction in system performance. Further, since signal resolution varies considerably, address reading errors occur frequently in the conventional AGC control method which employs averaging of all signals in analog mode, resulting in a reduction in system performance.